1. Field of the Invention
The invention relates to a deformable mirror, in particular a mirror which is utilized for a laser beam material machining apparatus.
2. Discussion of the Prior Art
A mirror of the general kind set forth is known from U.S. Pat. No. 5,777,807. Such mirrors have been successfully used for years in laser machining machines in order to be able to determine in the beam path directly downstream of the laser source the beam size or directly upstream of the optical focusing system the geometry and the spacing of the focal spot, and in that case to be able to compensate for influences of variable beam path lengths, as described in greater detail in CH 686 032 A5. If however an existing laser beam machining apparatus is to be only subsequently equipped with such a deformable mirror as a process-optimising optical system, then its rearward structural length which is governed by the linear actuator can project into the path of displacement of the handling devices for the workpieces, which causes trouble. Having regard to the fact that in practice operation is increasingly being implemented with laser beams of larger diameters, it would also be desirable not only to provide that the reflective mirror plate surface is bulged out convexly in an approximately spherical cap-like configuration in the relatively close area around the central point of application of axial force to the centre of the mirror plate, but also to achieve deformation which approximates as closely as possible to a sphere over a surface area which is as large as possible in the area around the centre point of the mirror plate.
Those aspects give rise to the present technical problem of so developing the deformable mirror of the general kind set forth, which in itself is tried and tested, in such a way that on the one hand a lower structural height is required for a predetermined stroke movement and on the other hand better spherically convex deformation is achieved with that stroke movement over even a relatively large mirror surface.
In accordance with the invention that object is attained by the aspect of the invention wherein the actuator is supported against the rear side of the mirror plate at mutually diametrally oppositely disposed positions which are displaced radially out of the longitudinal axis of the mirror.
In accordance therewith the translatorily acting actuator engages against the mirror plate at eccentric positions behind the mirror surface, said positions being in mutually diametrally opposite relationship with respect to the centre point of deformation. Because the edge of the mirror plate is held axially, that eccentric application of forcexe2x80x94with respect to the central onexe2x80x94results in a lever effect which, with the same axial stroke movement, results in a greater degree of curvature with respect to the edge of the plate, that is to say in the central region of the mirror plate, than the same stroke movement which however is applied directly behind the centre of the mirror plate. This means that, with the conditions otherwise being the same, a smaller structural length of the actuator is sufficient for the same deflection movement of the centre of the plate. In addition, the fact that the edge of the mirror plate is held axially to act in opposite relationship to the stroke movement now provides, in cross-section, in an enlarged central area, that the mirror surface adopts a curved configuration whichxe2x80x94compared to the central application of forcexe2x80x94is less parabolic than as desired in the form of a circular arc, from the centre of the mirror surface to laterally far beyond the region of the eccentric application of force.
The succession of the eccentric points of engagement of the actuator behind the mirror plate can extend continuously or discretely graduated along a track, that is to say, it can be in the form of a sequence of mutually adjacent thin pivots or in the form of a peripherally extending ring. The track of those points of engagement and thus the geometry of the axial cross-section of the ring do not have to be circular. An oval cross-sectional geometry (transversely with respect to the system axis through the centre point of the mirror plate) results in orthogonally different radii of curvature and in that respect the radius ratio 1:2 has the great advantage that, even with an angle of incidence of about 45xc2x0 for a 90xc2x0 beam deflection and in spite of a round mirror plate the arrangement practically eliminates astigmatism as a consequence of unavoidable compensation of focal length errors.